1. Field of the Invention
The present invention is related to a current-level controlling device for a power supply and related power supply, and more particularly, to a current-level controlling device and related power supply capable of regulating a reference voltage based upon variation of a peak value of a current sense signal, to make an actual voltage for activating over-current protection equal to an expected voltage for activating the over-current protection, to greatly improve the problem of time delay and voltage drift of the protection point.
2. Description of the Prior Art
Power supply is used to provide an electrical power source for operating an electronic device. According to the circuit architecture, power supplies can be classified into two types, Linear and Switching. A switching power supply has benefits of small volume, light weight and high power efficiency, so it can be widely used in various kinds of electronic devices, such as mobile phone, PDA, computer and peripherals, server and network appliances.
For sustaining the normal operation of the power supply, the protection mechanism of a control circuit for protecting the power supply is a very critical part (for example, protection functions of over-voltage, over-current, and over-power), and once the overload or the short condition happens, a power supply with complete protection functions can prevent the internal components or related appliances from being damaged.
Please refer to FIG. 1, which illustrates a schematic diagram of a switching power supply 10 of the prior art. The switching power supply 10 comprises the over-current protection function, and is used to convert an input voltage signal VIN to an output voltage signal VOUT with a proper voltage level. The switching power supply 10 comprises a transformer 100, a control unit 102, a comparator 104, a switch transistor Q1 and a current sensing resistor Rs. The operations of the circuit are stated as follows. First, the current sensing resistor Rs generates the current sense signal VCS based upon the primary winding current Id. Second, the comparator 104 compares the current sense signal VCS and a reference voltage VREF, and outputs an indication signal SOC to the control unit 102, such that the control unit 102 can determine whether it has fallen into the range of current protection. For example, when the current sense signal VCS is higher than the reference voltage VREF, the comparator 104 can indicate an over-current condition happens via the indication signal SOC, the control unit 102 can turn off the switch transistor Q1 to reduce current in the primary winding.
Simply speaking, the protection mechanism mentioned above is to compare the current sense signal VCS and the reference voltage VREF, such that the primary winding current Id can be controlled within a proper range for the purpose of protection. However, when the current sense signal VCS is higher than the reference signal VREF, the switch transistor Q1 cannot be turned off immediately owing to some non-ideal factors, and it will take an interval of time for the control unit 102 to turn off the switch transistor Q1. That is to say, there exists a time delay T_D, starting from the moment for the over-current condition being detected to the time for the switch transistor Q1 being turned off, and the current level right before being turned off will surpass the pre-defined level by a specific amount. In other words, the voltage level right before the over-current protection starts (abbreviated as “protection point voltage” hereafter) will be larger than the voltage level when the over-current condition is taking place. For different levels of the input voltage VIN, the voltage level of the protection point voltage varies accordingly.
For more details, please refer to FIG. 2, which illustrates the voltage difference of the protection point voltage for different input voltages within the same time delay. The input voltage VIN of the switching power supply 10 is proportional to the slope of the current sense signal VCS. Therefore, with the same reference voltage VREF, a higher input voltage VH will generate a current sense signal VCS of bigger slope, and a lower input voltage VL will generate a current sense signal VCS of smaller slope. Note that, a power supply always has the same time delay T_D since the time delay T_D is independent of the level of the input voltage VIN. As illustrated in FIG. 2, when the current sense signal VCS rises to the power limiting level corresponding to the reference voltage VREF, or the current sense signal VCS is greater than or equal to the reference voltage VREF, the comparator 104 transmits the indication signal SOC to the control unit 102, such that the switch transistor Q1 can be turned off. Since the circuit coming with the non-ideal factor, therefore, after the transmission delay T_D, the switch transistor Q1 starts being turned off, and the primary winding current Id can then be cut off. From the moment of the over-current condition being detected to the switch transistor Q1 being turned off, the input voltage VIN will continue to transfer power, such that the protection point voltage becomes VOPPH for the high input voltage VH, or the protection point voltage becomes VOPPL for the low input voltage VL. In other words, the protection point voltage will be higher than the reference voltage VREF, and as the input voltage VIN gets higher, the situation becomes even more obvious. Under this situation, when the input voltage VIN varies in a wide range, the protection point voltage will drift seriously, such that the output power levels corresponding to the high and the low input voltages differs a lot.